1. Field of the Invention
The invention relates principally to a conical sweep array antenna and a radar comprising such an antenna.
2. Description of the Prior Art
The book "Les Antennes, application aux radars et aux techniques spatiales" by Leo Thourel, second edition published by Dunod in 1971 describes, on pages 409, a flat antenna with a conical sweep. This book describes an antenna having groups of radiating slit guides. These guides are grouped in four identical quadrants fed by four excitation wave guides situated behind. Each of the quadrants forms an equiphase group, whose phase center is at the barycenter of the excitation amplitudes of said slits. Because of the identity of the four groups, the phase barycenters form the apex of a square whose center is the center of the antenna. If the four quadrants are fed in phase, the whole of the antenna is equiphase and the maximum radiation appears along the axis normal to the plane of the antenna, passing through its center. The conical sweep is achieved by feeding each of the quadrants through a phase shifter. The successive phase shift of the different quadrants allows a slope of the energy beam to be obtained.
The author emphasizes two serious defects of this device, first the level of the distant secondary loads is always very high and the gain factor is low. In fact, the diagram obtained is the product of the diagram of a quadrant multiplied by the alignment factor of the four barycenters which are always distant by more than a wave length. Second order lobes therefore inevitably appear (lobe of the arrays). In addition, the gain is reduced by the presence of these lobes and is affected by the losses in the phase shifters, which are often of the order of half a decibel, and which is deducted from the gain of the antenna alone.